In spite of the fact that the top surface of an engine cylinder block and the lower surface of an engine cylinder head may be smooth to the eye, the two surfaces never fit together closely enough to prevent leakage. Therefore, a gasket must be inserted between them to effect a seal. Various head gasket designs have been used, and these include metal head gaskets, fibrous head gaskets, sandwich type head gaskets using a metallic core and asbestos-bonded layers laminated to the core, among others. Sandwich-type gasket assemblies have sufficient resiliency to seal the cylinder head and sufficient heat resistance, especially when armored, to withstand the high temperature and pressure of the combustion chamber.
Many gaskets are formed from asbestos-bonded layers and other materials which have some porosity, hence the capacity to wick under certain circumstances. These gaskets tend to permit the wicking of fluid, such as oil and water, laterally through the porous gasket layers towards the peripheral edge of the gasket assembly. Normally this is not a serious problem from the standpoint of engine servicing or operation. However, in marine engines, once-through cooling is frequently used and the water circulated through the engine is frequently sea water or brackish water. Consequently, if wicking is allowed to occur, the cooling water and the dissolved minerals seep to the outside of the gasket assembly where the water evaporates leaving behind unsightly deposits. Furthermore, if these deposits are allowed to build up, they can cause corrosion.